Polling: a look inside the machinery of public opinion surveys

Science of probability

Once Newport and his colleagues formulate poll questions, the survey is sent to Tara McGhee, a survey design editor in Omaha. Ms. McGhee takes every path through the survey to make sure it's programmed correctly; in other words, depending on a respondent's reply – if he or she offers support for Romney or Obama, for example – that person is branched through the survey questions differently. Follow-up questions are prompted by their answers. She also checks spelling and grammar.

"We are expected to be perfect," says McGhee, who has been with Gallup for 10 years. "We are the last set of eyes on it. My personal mission is quality."

Stephanie Morrow, in the firm's nearby Lincoln, Neb., call center, is in charge of getting the teams of individuals in five offices – including Houston, where the Spanish-language outreach is done – ready to make their calls. About 100 interviewers worked the June political poll. As the survey got started – it ran for four nights, from Thursday, June 7, to Sunday, June 10 – Ms. Morrow and her team did random quality checks, listening to recordings of the calls.

As it has done for years, Gallup outsources a critical part of the polling process – sampling – to a firm called Survey Sampling International, which has an office in Connecticut.

This is an element of the undertaking that most respondents and citizens are blind to – and it's vital. Sampling is the process of finding a random group of Americans – 1,004 in the case of the June Gallup poll – whose responses represent the same views that would be obtained if every American adult were interviewed. (Sample sizes do vary among reputable outfits and can be smaller, though experts believe the number used by Gallup, and others, is a good one.)